Flexible field-effect transistor arrays with patterned solution-processed organic crystals

Organic field-effect transistors (OFETs) have attracted interest because they are the fundamental building blocks of advanced electronics applications such as active-matrix displays,^1–3 radiofrequency identification tags,^4,5 and sensors.^6,7 The development of OFETs is characterized by several major factors, including the improvement of device performance, reduction of manufacturing costs, and extension to more applications. OFETs with solution-processed organic crystals as active semiconductor materials on flexible substrates are promising candidates that meet such requirements. Given that the solubility of organic molecules enables device fabrication by relatively simple solution-based technologies, solution-processed organic crystals are ideal materials that can be used to achieve high device performance because of their highly ordered molecules, lack of grain boundaries, and substantial reduction in production costs.^8–18 Electronic systems that can be processed in a largely covered area on flexible substrates have been developed because such systems can greatly extend applications to classes beyond the scope of rigid wafer-based electronics.^19–23 Although studies have been devoted to solution-processed OFETs on flexible substrates, current solution processes involved in the growth of organic semiconductor crystals on flexible substrates do not have a patterning capability, which is a desirable integration with commercial semiconductor technology.^{8–10,24–31} Furthermore, traditional lithography technologies are not applicable to soluble organic films. Patterning techniques that use self-assembled molecules have been proposed to develop OFET arrays on flexible substrates, but polycrystalline organic materials in the channel regions are used.³² Thus, fabrication of transistor arrays with patterned organic crystals via a solution-based method using flexible substrates is a great challenge.

To solve this issue, we use a direct spin-coating process from a mixture of a small-molecule semiconductor and a polymer insulator on a flexible plastic substrate with patterned wettability. In this process, large plate-like organic crystals are formed in the patterned regions during spin coating without any post-treatment. The resulting FET arrays show high electrical performance, indicating that the proposed method is a promising technology that can be applied to develop solution-processed organic crystals in flexible electronics.

The proposed method involves a simple and efficient fabrication of patterned organic crystals from solutions under ambient atmosphere. The fabrication process of flexible OFETs with patterned organic semiconductor crystals is illustrated in Fig. 1. Polyethylene naphthalate (PEN) was used as the flexible plastic substrate. We initially evaporated an Au layer with a thickness of 50 nm in a shadow mask to form gate electrode arrays on the PEN substrate. A CYTOP thin film (540 nm) was deposited from the solution by spin-coating. The substrate was then baked for 10 min at 90 °C to protect the plastic substrate from damage. The fluoropolymer of CYTOP formed a highly hydrophobic surface. Surface patterning was performed by exposing the CYTOP-covered PEN substrate to O₂-plasma treatment through a shadow mask. This treatment can partially remove the CYTOP layer and change the hydrophobic patterned region to a hydrophilic patterned region. The patterned squares had the following dimensions: length, 0.5 mm; width, 0.2 mm; and depth, 40 nm. After surface wettability was patterned, a mixture of dioctylbenzothienobenzothiophene (C8-BTBT, Nippon Kayaku Ltd.) and polymethylmethacrylate (PMMA, Sigma-Aldrich) in anisole was spin-coated on the substrates. The p-type small molecule of C8-BTBT has excellent solubility and is a promising material that can be used to achieve high device performance.^18,33–35 Given that the modified surface regions were hydrophilic to solutions containing organic semiconductors, small droplets were confined in the square regions. Thus, this selective deposition of active materials isolated the devices from one another. In the final step, molybdenum oxide (MoO₃) and Au were evaporated to allow the source and drain electrodes to form a bottom-gate, top-contact device structure. The inserting layer of MoO₃ can modify the organic/metal interface to exhibit a more efficient charge injection.³⁶ Finally, 300 nm parylene was coated on the sample as a protective layer.

The OFET arrays on the flexible PEN substrate obtained by the proposed process are shown in an optical image and a micrograph in Figs. 2(a) and 2(b), respectively. Spin-coating of C8-BTBT and PMMA mixture on the substrate with patterned wettability produced large plate-like C8-BTBT crystals in the patterned regions. Figure 2(c) shows a cross-polarized microscopy image of a single device containing organic crystals in the channel. The grain sizes of crystals were as large as hundreds of microns. Strong birefringence under cross-polarized light was observed from the crystals, indicating their crystalline nature. After O₂-plasma treatment was performed, the remaining CYTOP film (500 nm) was applied directly as the bottom gate dielectric material. This polymer insulation layer allows the device arrays to become flexible.³² However, fixed interface charges can be introduced during plasma treatment.³⁹ This issue can be solved by phase separation during spin-coating, thereby forming a PMMA-bottom C8-BTBT-top double layer. Our previous results revealed that organic crystals are formed because of the strong molecular interaction of C8-BTBT with the aid of PMMA spreading effect; C8-BTBT/PMMA interface also have excellent morphological and electrical properties.^8,9

The transfer and output characteristics of a typical OFET are shown in Figs. 3(a) and 3(b), respectively. The pattern of semiconductor organic crystals is favorable to manufacture OFETs commercially because this technique eliminates the parasitic current paths between neighboring device elements in an array. In Fig. 3(a), gate leakage current was considerably reduced to 10^–11 A, and a high on-off ratio of >10⁶ was obtained. The device exhibited good electrical performance with a high field-effect mobility (μ_FET) of 0.22 cm² V^–1 s^–1, a threshold voltage (V_T) of 1.3 V, and a sub-threshold slope (SS) of 0.7 V/dec. In Fig. 3(b), the output curves presented a non-linear behavior in the drain current passing in the low drain-voltage region. This behavior is due to the contact resistance effect. Given that the highest occupied molecular orbital of C8-BTBT is 5.7 eV, a bottom-gate top-contact architecture that involves access resistance from metal/organic interface to the conducting channel was applied in our device arrays. Although we applied an inserting layer of MoO₃ between the Au electrode and the C8-BTBT crystals as observed in the output curves, an ideal charge injection with perfect energy level alignment was still difficult to achieve. In the high drain-voltage region, the drain current showed good saturation characteristics. This result indicated an ideal transistor operation.

The characteristics of the transistor being bent for the first time and after bending for ten times are shown in Fig. 4. The devices were bent to a radius as small as 6 mm. Mobility decreased from 0.22 cm² V^–1 s^–1 to 0.06 cm² V^–1 s^–1 as the first bending, while it was maintained as 0.06 cm² V^–1 s^–1 after the transistor was bent 10 times. Table I shows slight shifts in V_T, and no significant changes in SS and on/off ratio of the device before bending, being bent for the first time, and after bending for ten times. This result indicates that the property of C8-BTBT/PMMA interface was not influenced by bending.^8,9,37,38 The solution-processed C8-BTBT crystal transistors exhibited good stability in performance during the bending test after an acceptable decrease in mobility. Therefore, our devices can be potentially used in flexible electronics applications.

The obtained flexible FET arrays with solution-processed organic crystals can be used to develop organic electronics with several advantages. First, the proposed device fabrication involves a low-cost, high-throughput, simple, and efficient spin-coating process and can be used for the deposition of organic semiconductors. Thus, the use of this method can eliminate the need for expensive equipment such as in vacuum vapor deposition or inkjet printing. Second, organic crystals function as active channel materials, which are needed to achieve high device performance because these crystals have highly ordered molecular packing without grain boundaries that can scatter charge carriers. The patterning technique of our method enables selective deposition of organic semiconductor crystals in well-defined geometric features. This technique can also be used to improve organic crystals and develop a more feasible technology. Finally, flexible devices are of significant interest because of their potential applications such as flexible displays and sensors, which are not achievable when devices on rigid substrates are used.

The mobility values obtained from our flexible device arrays are lower than those reported by Minemawari et al.³⁵ Therefore, the obtained mobility of our devices does not represent the intrinsic mobility of C8-BTBT. This mobility can be attributed to the contact resistance effect as indicated by a non-linear increase in drain current of output curves. Thus, a more suitable contact material is necessary to improve our device further. We also observed that the spreading effect of PMMA is important to form plate-like organic crystals and simultaneously provide a horizontal force, resulting in a larger distance between C8-BTBT molecules in the crystals. Thus, the molecular packing of organic crystals depends on processing methods.^35,40 A mechanical shearing technique has been applied to triisopropyl pentacene, in which an improved mobility is obtained from a resulting strained lattice.⁴⁰ To improve our method further, an enhanced fabrication process that can minimize the influence of PMMA spreading on organic molecular packing is currently under investigation.

In conclusion, a fabrication method used to develop flexible FET arrays based on patterned organic crystals was demonstrated using a simple and efficient solution-assisted process. The organic crystals were well confined in patterned regions. Large plate-like organic crystals were then formed. Thus, high device performance was obtained from the fabricated FET arrays, indicating that the proposed technology could be used in practical applications of flexible electronics.

For their fruitful discussions, the authors would like to express their appreciations to Prof. Kazuo Takimiya, Dr. Songlin Li, and Dr. Haisheng Song. The authors also would like to thank Nippon Kayaku for supplying C8-BTBT. This study is supported partially by 973 project under Grant No. 2013CBA01600, NSFC under Grant Nos. 61229401, 61076017, and Core Research for Evolutional Science & Technology (CREST) project from Japan Science and Technology Agency (JST).